This invention relates to a method of manufacturing aluminum alloy sheets excellent in hot formability, i.e. a property of exhibiting very high ductility and very low deformation resistance in a hot atmosphere enough to enable forming same by blow forming as employed in the forming of sheet plastic.
Heat treatable aluminum alloys in general include Al-Cu alloys, Al-Cu-Mg alloys, Al-Mg-Si alloys, and Al-Zn-Mg-Cu alloys. These aluminum alloys are generally equivalent to aluminum alloys numbered 2000's, 6000's and 7000's according to JIS and AA (Aluminum Association of U.S.A.).
A typical conventional method of manufacturing aluminum alloy sheets from such heat treatable aluminum alloys comprises hot rolling an ingot, which has been homogenized at a temperature of 460.degree. to 560.degree. C., at substantially the same temperature as the homogenizing temperature, into a hot rolled plate having a thickness of 2 to 10 mm (usually 6 mm), cold rolling the hot rolled plate with a reduction ratio of 20% or more into a cold rolled sheet having a thickness of 1 to 5 mm, and further cold rolling the cold rolled sheet with a reduction ratio of 20 to 80% into a final thickness of 0.5 to 3 mm. If required, the first cold rolled sheet may be subjected to intermediate annealing to remove internal stresses or working stresses in the cold rolled sheet to thereby obtain an "0" temper material. The intermediate annealing is conducted under such conditions that the sheet is heated at a temperature of about 413.degree. C. in a manner slowly heating the sheet and then slowly cooling same at a cooling rate of about 28.degree. C./hr while the sheet is cooled from 413.degree. C. to 260.degree. C., as already known from "Aluminum Standards and Data," published by The Aluminum Association (1984), or under similar conditions. The heating temperature and the cooling rate are controlled such that most of the working hardening and the precipitation hardening which would take place before the intermediate annealing can be removed and no further precipitation hardening can take place.
However, cold rolled aluminum alloy sheets thus obtained by the conventional method suffer from coarse crystal grains, that is, the crystal grain size usually shows a range of 100 to 300 .mu.m when it is measured in the direction of cold rolling (The "crystal grain size" hereinafter referred to also means one obtained in the same measuring manner as above). Even if the cold rolled sheets are subjected to final annealing or solution heat treatment in order to recrystallize them, the minimum recrystallized grain size is of the order of 20 .mu.m. An aluminum alloy sheet with such grain size cannot show hot formability as high as that of superplastic aluminum alloys.